1. Technical Field
This invention relates in general to spectrophotometric systems including sensors for non-invasively determining biological tissue oxygenation utilizing near-infrared spectroscopy techniques, and in particular to a device for calibrating such systems and for evaluating the proper functioning of such systems.
2. Background Information
Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation levels. The NIRS method is based on the principle that light in the near-infrared range (700-1000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near-infrared range has specific absorption spectra that vary depending on its oxygenation state; i.e., oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and by measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin and deoxyhemoglobin can be monitored. The ability to continually monitor cerebral oxygenation levels, for example, is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death.
NIRS-type sensors typically include at least one light source and one or more light detectors for detecting reflected or transmitted light. The light signal is created and sensed in cooperation with an overall NIRS system that includes a monitor portion having a computer or processor that runs an algorithm for processing signals and the data contained therein. Typically the monitor portion is separate from the sensor portion. Thus; the sensor and monitor portions comprise the overall NIRS system. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1000 nm are typically used. A photodiode or other light detector is used to detect light reflected from or passed through the tissue being examined. The NIRS system processor cooperates with the light source and detector to create, detect and analyze the signals in terms of their intensity and wave properties. U.S. Pat. Nos. 6,456,862, and 7,092,701, both of which are hereby incorporated by reference in their entirety and are commonly assigned to CAS Medical Systems, Inc., of Branford, Conn., the assignee of the present patent application, disclose an NIRS system (e.g., a cerebral oximeter) and a methodology for analyzing the signals within the NIRS system.
Oftentimes an NIRS system typically includes a sensor portion having a plurality of discrete light sources located together in one location along with one or more light detectors disposed at predetermined distances from each other and from the light source. Each light source produces an infrared light signal at a particular wavelength at which a known absorption response is produced depending on the amount of oxygen concentration in the hemoglobin. Several different wavelengths are typically employed, for example, at 780 nm, 805 nm, and 850 nm. Ideally, the light sources would operate at specific wavelengths that do not vary at all from one NIRS system to another, nor would the wavelength of light output from an individual light source vary over time. However, in practice the light output from a discrete light source varies from device to device due to, e.g., manufacturability and material constraints and variables, and also over time due to operating variables such as, e.g., temperature.
What is needed, therefore, is a device that can be used to accurately and quickly calibrate an NIRS system and to evaluate the proper functionality of the NIRS system.